Sitting in the back of a vehicle with a seatbelt but just a metal loop where the buckle should be on the seat, trying to work out the best way to wrap things in case the vehicle hits something at 60 MPH.

Can't produce the name of what I've done, but it's the "shove a loop inside another loop" thing, and then rolled down the slack. I think it'll stay taut for a good 200-400ms, which is probably all I need. Other ideas? (CC @gdkar )